Existing aircraft typically include two or more primary engines for propulsion. These aircraft also typically include at least one APU that provides electrical and/or pneumatic power in addition to or in lieu of the power provided by the primary engines. Accordingly, APUs can be used to provide power to the aircraft when the primary engines are not running, for example, when the aircraft is on the ground at an airport gate. The APUs can also provide temporary power to start the primary engines during normal operations, and/or temporary emergency power during an engine-out condition or other emergency condition during flight operations. Additionally, the APU can be used during flight to provide additional electrical and/or pneumatic power whenever the aircraft systems require an amount that exceeds the electrical and/or pneumatic power capability of the primary propulsion engines.
There are certain instances when an existing certificated aircraft undergoes a design change so as to adapt the airframe to an entirely different purpose for which it was originally intended. For example, a civil transport aircraft may be subsequently adapted to perform military or police surveillance missions. Such aircraft modifications to a baseline aircraft are colloquially termed “derivative aircraft”. A derivative aircraft is thus often accompanied by a need to provide a substantial increase to the electrical power requirements due to the on-board presence of additional electrical and/or electronic equipment (e.g., on-board surveillance cameras, specialized radar, computers and associated equipment) carried by the aircraft to perform the missions for which it is designed. This increased electrical power requirement is often greater than the maximum electrical output for which the original APU was designed. The traditional solution to additional electrical power requirements is to replace the original APU with a larger APU having a correspondingly greater electrical power output capacity. However, such a larger APU will in turn usually require airframe space that is not readily available in the baseline (unmodified) aircraft, thereby requiring a major structural design change or even relocating the APU to another area within the airframe.
It would therefore be especially desirable to provide a solution to the problems described above whereby increased electrical power output could be provided to accommodate an increased power requirement of a baseline aircraft without the need to relocate and/or replace the existing APU. It is towards fulfilling such needs that the present invention is directed.